How To Resolve ORA-00979 Not a GROUP BY Expression

Have you tried to run a query and got the “ORA-00979: not a group by expression” error? Learn what it is and how to resolve the error in this article.

ORA-00979: not a group by expression

The ORA-00979 error happens when you have at least one column in your SELECT clause that is not in your GROUP BY expression when you are using an aggregate function. Common aggregate functions include SUM, AVG, MIN, MAX, and COUNT. Any column or expression in your SELECT clause must also be listed in the GROUP BY clause.

Here’s an example of a query that will generate the error:

SELECT first_name, last_name,COUNT(*)FROM student
GROUPBY first_name;

SELECT first_name, last_name, COUNT(*)
FROM student
GROUP BY first_name;

Result:

ORA-00979: not a GROUP BY expression

Why Do I Get The ORA-00979 Error?

This error happens because you’re using an aggregate function, and there is at least one column in the SELECT clause that is not in the GROUP BY clause.

Using the example query above:

SELECT first_name, last_name,COUNT(*)FROM student
GROUPBY first_name;

SELECT first_name, last_name, COUNT(*)
FROM student
GROUP BY first_name;

Because I use an aggregate function (COUNT), I need to define all of the columns in a GROUP BY clause that are in the SELECT clause.

In this example, I have specified the first_name and last_name column in the SELECT clause, but the last_name column is not in the GROUP BY clause.

Even if I have some fields in the GROUP BY clause, if I don’t specify all of the fields from the SELECT clause, I’ll still get an error.

Because, if you don’t have a GROUP BY but you want to SELECT the column, Oracle doesn’t know what value to show when using this aggregate function. Should it show the first value? The last value? A random value?

How To Resolve the ORA-00979 Error

To resolve the ORA-00979: not a group by expression error, simply ensure that all of the GROUP BY columns match the SELECT clause.

Conclusion

So, in conclusion, the ORA-00979: not a group by expression error happens because the columns in the SELECT clause don’t match the columns in the GROUP BY clause. To resolve the error, make sure the columns match.

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